The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 291

harvesting and storage, or during homogenization, emulsification, or dispersion of
food ingredients must initiate many biochemical reactions, whereby especially those
reactions involving transfer of single electrons may be uncoupled resulting in the
release of free radicals. In aerobically respiring living cells, most of the oxygen
absorbed is catalytically reduced to water by cytochrome c oxidases or blue copper
oxidases in the respiration cycle, but up to 5% of the total O 2 may be only univalently
reduced to O - . 2 In fresh postharvest foods and certainly in those stored and pro-
cessed, the tight compartmentalized control of active oxygen species, including
superoxide (O · - ) and hydrogen peroxide (H 2 O 2 ) is gradually lost. Thus, under these
conditions free radical intermediates including active oxygen species may accumu-
late. In complex food systems it has not yet proved possible to always define in
precise chemical terms how such released free radicals react with the very large
number of compounds present in food materials, although it is now becoming
increasingly accepted that free radicals damage the main quality attributes — texture,
flavor, and color. Some enzymes like peroxidases and lipoxygenases are often able
to initiate oxidation directly through the involvement of free radicals. Some of these
enzymes can also withstand thermal processing and therefore in processed foods,
where other protective enzymes have been denatured, they may still be sufficiently
active to initiate changes in quality during long-term storage. Plant cultivars have
been identified that lack a particular isoenzyme and, for example, plants grown from
seeds lacking two lipoxygenase isoenzymes showed no obvious deleterious effects. 3
The literature abounds with studies on oxygenases in a wide range of fruits and
vegetables including potatoes, avocados, olives, bananas, mangoes, tea, coffee,
cocoa, and some exotic species. However, there is a need for detailed studies at the
molecular level to determine both the precise reasons for the occurrence of such a
wide range of isooxygenases within a given plant species and the relative mode of
action of the isoenzymes on various natural substrates. During fruit development
and ripening, the activity of many enzymes including peroxidases, polyphenol oxi-
dases, and lipoxygenases, as well as the concentration of their substrates, changes
dramatically. Peroxidases may play important roles in the color changes associated
with the ripening of fruits and the senescence of fruits and vegetables. For instance,
it has been claimed 4,5 that peroxidase activity and concentrations of hydrogen peroxide
increase during the senescence and ripening of fruits. Abeles et al. 6 reported studies
that implicate the biosynthesis of a 33-kDa peroxidase during ethene-induced senes-
cence in cucumber cotyledons. As fruits age enzymes also become more soluble and
activation can occur. During senescence colored compounds are formed, which are
not the direct result of the primary oxidation reaction. The formation of such colored
compounds, often brown or black, is most obvious in products and spoiled foodstuffs
and is initiated by cellular damage and release of previously bound enzymes.
LIPOXYGENASES
O CCURRENCE
Lipoxygenase (EC 1.13.11.12, linoleate: oxygen oxidoreductase) is an iron-containing
dioxygenase which catalyzes the oxidation of polyunsaturated fatty acids containing